Apparatus and process for generation of energy by organic rankine cycle

ABSTRACT

An apparatus ( 10 ) and a process for generation of energy through organic Rankine cycle (ORC) in which the heat exchanger ( 70 ) is of the hairpin type, so as to make the apparatus more flexible and optimise the cycle efficiency. The hairpin heat exchanger is able to stably carry out the preheating, once—through evaporation and superheating steps both at the nominal load and at the partial and transitory loads, either for sub-critical ORC cycles or for super-critical ORC cycles.

TECHNICAL FIELD

The present invention relates to an apparatus and process for energy generation by organic Rankine cycle.

BACKGROUND ART

Apparatuses based on a thermodynamic Rankine cycle are known that convert thermal energy into mechanical and/or electric energy in a simple and reliable manner. In these (ORC) apparatus working fluids of the organic type (of high or medium molecular weight) are preferably used in place of the traditional water/vapour system, because an organic fluid is suitable for conversion of heat sources at relatively low temperatures, generally between 100° C. and 300° C., but also at higher temperatures, in a more efficient manner. The ORC conversion systems therefore have recently found increasingly wider applications in different sectors, such as in the geothermic field, in the industrial energy recovery, in apparatus for energy generation from biomasses and concentrated solar power (CSP), in regasifiers, etc.

An apparatus of known type for conversion of thermal energy by an organic Rankine cycle (ORC) generally comprises: at least one heat exchanger exchanging heat between a high-temperature source and a working fluid, so as to heat, evaporate and superheat the working fluid; at least one turbine fed by the working fluid in the vapour phase coming out of the heat exchanger so as to carry out conversion of the thermal energy present in the working fluid into mechanical energy according to a Rankine cycle; at least one generator operatively connected to the turbine, in which the mechanical energy produced by the turbine is converted into electric energy; at least one condenser where the working fluid coming out of the turbine is condensed and sent to at least one pump; from the pump the working fluid is fed to the heat exchanger.

ORC cycles and related apparatus are known in which evaporation is sub-critical. Reproduced in FIGS. 2 a and 2 b is a typical Rankine cycle, obtained with an organic fluid, by sub-critical evaporation. The organic fluid is pumped by the pump from pressure of point 1 (pump suction) to pressure of point 2 (pump delivery). From point 2 the fluid is heated until point 3. In the most general case heating contemplates the sensible-heat exchange with the working fluid in the liquid phase (from 2 to 2′), the latent-heat exchange between saturated liquid and saturated vapour (2′ to 2″), the sensible-heat exchange with vapour (2″ to 3). When point 3 has been reached, the fluid is introduced into the turbine. The exit conditions out of the turbine are represented by point 4. From point 4 the fluid is cooled to point 5 and condensed until point 1. In known apparatus with sub-critical evaporation, heating of the organic fluid passes through three different sections: preheating, evaporation and superheating (in some cases superheating can be absent). In these known apparatus different heat exchangers are normally used for thermal exchange of sensible heat and latent heat. The heat exchanger of known apparatus therefore comprises a preheater, an evaporator and, optionally, a superheater. This because usually a big volume is required for the evaporator as generally the vapour of a fluid has a specific volume much bigger than the liquid. In addition, large exchange surfaces are required to make the vapour acquire sensible heat because the heat exchange coefficients of vapour are very low.

Document WO 2011/012516 of known art illustrates a steam generator including tubes passing through the generator, from a water inlet to an superheated steam outlet, disposed horizontally in banks perpendicularly passed through by fumes.

Document U.S. Pat. No. 4,627,386 of known art shows a steam generator or boiler in which the thermal energy used for generating steam is obtained from a gas turbine.

In document JP 60 251388 of known art the exhaust gases of a gas turbine are introduced into a heat exchanger and heat exchange is carried out in an evaporator. The heat exchange tubes of the evaporator are disposed horizontally in groups.

Document WO 2006/060253 of known art depicts a method and an apparatus using an organic Rankine cycle for generating energy on a sea boat. The method comprises the following steps: providing an ORC device including at least one evaporator, a turbogenerator, a condenser and a cooler feeding pump; arranging the evaporator within an exhaust duct of a power plant of a sea boat; setting the power plant in operation and selectively pumping cooler through the ORC device.

DISCLOSURE OF THE INVENTION

Within this scope, the Applicant has aimed at improving known plants under different points of view, in particular in relation to optimisation of the apparatus intended for heat exchange, based on the nature of the organic fluid used.

In greater detail, the Applicant has aimed at optimising the apparatus carrying out change of state, from liquid to vapour, of the organic liquid used.

The Applicant has found that adoption of a heat exchanger of the “hairpin” type makes the apparatus more flexible because with this exchanger once-through evaporation is carried out which does not require sub-cooling of the fluid entering the evaporator, which would be necessary in the pre-heater and boiler configuration. In addition, this exchanger makes the starting and turning off operations of the apparatus more flexible, because it can remain in operation under dry-running conditions, i.e. with the primary side started and the secondary side dry.

More particularly, the invention relates to an ORC apparatus for energy generation through the organic Rankine cycle comprising: an organic working fluid; at least one heat exchanger to exchange heat between a high temperature source and the working fluid, so as to heat, evaporate and superheat said working fluid; at least one turbine fed with the working fluid in the vapour phase coming out of the heat exchanger, to carry out conversion of the thermal energy present in the working fluid into mechanical energy according to a Rankine cycle; at least one condenser where the working fluid coming out of said at least one turbine is condensed and sent to at least one pump; the working fluid being then fed to said at least one heat exchanger; characterised in that the heat exchanger is of the hairpin type and comprises at least one inner tube surrounded by an outer jacket; wherein the inner tube and outer jacket extend along at least two rectilinear stretches mutually connected by at least one curvilinear stretch.

In another aspect, the present invention relates to an ORC process for energy generation through the organic Rankine cycle, comprising: i) feeding an organic working fluid through at least one heat exchanger to exchange heat between a high temperature source and said working fluid, so as to heat, evaporate and superheat said working fluid; ii) feeding the organic working fluid in the vapour phase coming out of the heat exchanger to at least one turbine to carry out conversion of the thermal energy present in the working fluid into mechanical energy according to a Rankine cycle; iii) feeding the organic working fluid coming out of said at least one turbine to at least one condenser where the working fluid is condensed; iv) sending the organic working fluid coming out of the condenser to said at least one heat exchanger (30); characterised in that step i) comprises: making the organic working fluid flow through a heat exchanger of the hairpin type comprising at least one inner tube surrounded by an outer jacket; wherein the inner tube and outer jacket extend along at least two rectilinear stretches mutually connected by at least one curvilinear stretch.

By the term “hairpin” it is intended a heat exchanger comprising one or more inner tubes inserted into an outer shell in which the inner tubes and outer shell extend along rectilinear stretches mutually connected by curvilinear stretches, like a street with “hairpin” bends. A first fluid flows in the inner tubes and a second fluid flows between the inner tubes and outer shell.

This type of exchanger, also referred to as “double-tube exchanger” is known by itself in the technical literature. For instance, the text “Process Heat Transfer, Principles and Applications”, by Robert W. Sert, published in April 2007 by Elsevier Science & Technology Books (ISBN: 0123735882) at pages 3/86 and 3/87 describes the hairpin exchanger as provided with an inner tube or a bundle of inner tubes, surrounded by an outer tube and in which both the inner tube and outer tube extend as a tube coil formed with at least two rectilinear stretches connected by a curved stretch.

Using the hairpin heat exchanger inside the ORC cycle, said heat exchanger is able to carry out a state conversion from liquid to superheated vapour by a single apparatus, enabling the sizes of the whole plant and the industrial spaces dedicated thereto to be reduced.

Since the hairpin heat exchanger further is of easy manufacture, limited cost and high reliability, it helps in making the whole plant cheaper and more reliable.

The hairpin heat exchanger is able to stably carry out the preheating, once-through evaporation and superheating steps both at nominal load and at partial and transitory loads, for sub-critical ORC cycles and also super-critical ORC cycles. By “once-through” evaporation it is intended a process in which physical distinction between preheater, evaporator and superheater is not provided, but the fluid goes on without a break from the starting liquid state to the final superheated vapour state. As a result, the plant can be used with different organic fluids and optimised as a function of the nature of same.

The hairpin heat exchanger performing all the above mentioned exchange steps in a single tube without a break is consequently also self-draining during the turning-off step.

In addition, the exchanger of the hairpin type is able to come into operation under dry-running conditions. The term “dry-running conditions” is understood as indicating the conditions according to which the only hot side of the exchanger is fed with the fluid. Using the hairpin exchanger for carrying out a once-through evaporation it is possible to supply the diathermic oil alone on the skirt side and subsequently supply the organic fluid on the cold side.

The configuration of the hairpin type further has the advantage of enabling heat exchange with great temperature differences between fluid entry and fluid exit, i.e. with high thermal lengths, the mechanical stress being low. In fact, using this geometry, it is possible to uncouple the expansion on the outer shell from the expansion of the tubes.

The hairpin heat exchanger is able to withstand high temperature differences, even beyond 100-200° C., between the incoming heating fluid (FIGS. 2 b and 3 b, point A) and outgoing heating fluid (FIGS. 2 b and 3 b, point B).

Preferably, the hairpin heat exchanger is of the countercurrent type, with or without buffers.

Preferably, the hairpin heat exchanger comprises an inner-tube bundle surrounded by a jacket.

According to a preferred embodiment, the hairpin heat exchanger comprises a single inner tube surrounded by a jacket.

In accordance with a preferred embodiment of the process, in step i) heating of the organic working fluid is of the super-critical type.

In accordance with a preferred embodiment of the process, in step i) heating of the organic working fluid is of the sub-critical type.

The advantage of performing the cycle making a selection between sub-critical evaporation and super-critical evaporation resides in optimising the conversion performances from thermal energy into electric energy. The operating conditions optimising the thermal cycle performances, such as pressure of the evaporation, depend on the fluid nature. By changing the type of organic fluid used, there is also a change in the process parameters optimising the cycle efficiency, and consequently in the nature of the evaporation that can be sub-critical or super-critical.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages will become more apparent from the detailed description of a preferred but not exclusive embodiment of an apparatus and a process for energy generation through the organic Rankine cycle in accordance with the present invention.

The detailed description of these configurations will be set out hereinafter with reference to the accompanying drawings, given by way of non-limiting example, in which:

FIG. 1 diagrammatically shows the base configuration of an apparatus for energy generation through the organic Rankine cycle according to the present invention;

FIGS. 2 a and 2 b respectively show an organic Rankine cycle (ORC) with sub-critical evaporation and diagram T-Q reproducing the conversions taking place in the evaporator;

FIGS. 3 a and 3 b respectively depict an organic Rankine cycle (ORC) with super-critical evaporation according to the present invention and diagram T-Q reproducing the conversions taking place in the evaporator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

With reference to the mentioned figures, generally denoted at 10 is an apparatus for energy generation through the organic Rankine cycle (ORC) according to the present invention.

Apparatus 10 comprises an endless circuit in which an organic working fluid flows which has a high or medium molecular weight. This fluid can preferably be selected from the group comprising hydrocarbons, fluorocarbons and siloxanes.

FIG. 1 shows the circuit of the Rankine cycle in its base configuration and contemplates: a pump 20, a heat exchanger 30, a turbine 40 connected to an electric generator 50, a condenser 60.

Pump 20 admits the organic working fluid from condenser 60 into the heat exchanger 30. In the heat exchanger 30 the fluid is heated, evaporated and then fed in the vapour phase to turbine 40, where conversion of the thermal energy present in the working fluid into mechanical energy and then into electrical energy through generator 50 is carried out. Downstream of turbine 40, in condenser 60, the working fluid is condensed and sent again to the heat exchanger through the pump 20.

Pump 20, turbine 40, generator 50 and condenser 60 will be not further described herein as they are of known type.

Advantageously, the heat exchanger 30 is of the “hairpin” type, i.e. it comprises a single inner tube or several inner tubes (tube bundle) 70 in which circulation of the organic working fluid occurs. Tubes 70 are inserted in an outer shell/skirt/jacket 80 and between the tubes 70 and shell 80 a hot fluid, diathermal oil for example, is caused to flow. The inner tubes 70 and outer shell 80 extend along rectilinear stretches 70 b, 80 b connected to each other by curvilinear stretches 70 a, 80 a.

In the non-limiting diagrammatic example shown, the hairpin heat exchanger 30 comprises a U-shaped inner tube 70 having two rectilinear stretches 70 b connected by a curvilinear connecting stretch 70 a. The inner tube 70 extends inside the outer shell 80 that will takes the same U-shaped configuration with two rectilinear stretches 80 b connected by a curvilinear connecting stretch 80 a. A first end 90 (inlet) of the inner tube 70 is in fluid connection, through suitable pipeline, with pump 20. A second end 100 (outlet) of the inner tube 70 is in fluid connection, through suitable pipeline, with turbine 40. In the vicinity of the second end 100 of the inner tube 70, the outer shell 80 has an inlet 110 for the hot fluid and, in the vicinity of the first end 90 of the inner tube 70, the outer shell 80 has an outlet 120 for said hot fluid. The organic working fluid flows from the first end 90 to the second end 100 while the hot fluid runs from inlet 110 to outlet 120, so that the heat exchanger 30 shown works in counter-current. According to variants not shown, the heat exchanger 30 can have a bundle of inner tubes 70 and/or work in the same current direction and/or have “n” rectilinear stretches connected by “n-1” curvilinear stretches.

In accordance with the process of the invention, the working fluid running in the hairpin heat exchanger 30 passes without a break from the initial liquid state to the final state of superheated vapour. Evaporation takes place in the absence of contact between liquid and vapour and therefore under the so-called “once-through” condition.

In apparatus 10 according to the invention it is possible to carry out ORC processes either with sub-critical heating/evaporation or super-critical heating/evaporation.

FIGS. 2 a and 2 b describe the heat exchange during heating of the organic fluid in the more general case of sub-critical heating. During heat exchange the hot fluid (diathermic oil, for example) entering at point A is cooled by transfer of heat Q until it reaches the conditions of point B. The organic fluid coming out of pump 20 at the described conditions from point 2 absorbs heat Q and is heated. The thermal profile followed by the fluid during heating is reproduced by curve 2-2′-2″-3 in FIG. 2 a.

Reproduced in FIGS. 3 a and 3 b is an organic Rankine cycle, ORC, with super-critical evaporation. Unlike the evaporation described in FIG. 2 a, the fluid is pumped by the pump until a pressure higher than the critical one. By heating from this point until point 3 it is not possible to identify points 2′ and 2″ characterising the phase transition. In particular, the specific fluid volume changes continuously, without discontinuity from liquid to vapour. This is true at the nominal pressure, but it should be pointed out that during the starting and turning-off transients, crossing of the sub-critical region is unavoidable.

In super-critical ORC cycles heating takes place without phase changes, however in the starting and turning-off transients and during the consequent pressurisation/depressurisation transients the saturation curve is crossed and therefore particular care and attention is paid to systems adapted to avoid formation of liquid pockets in areas where there is the presence of superheated vapour.

The conversion of state from liquid to vapour in the single hairpin exchanger is able to exchange both the sensible heat necessary to bring the fluid to conditions of saturated liquid (preheating, PH, FIG. 2 a, stretch 2-2′), and the latent heat for bringing the saturated liquid to the conditions of saturated vapour (evaporation, EV, FIG. 2 a stretch 2′-2″), as well as the sensible heat necessary for vapour superheating (superheating, SH, FIG. 2 a stretch 2″-3). The thermal energy exchanged in the apparatus with hairpin exchanger according to the invention enables the fluid to carry out conversions either involving sensible and latent heat (sub-critical conditions, see FIG. 2 a) or involving heat exchange under super-critical conditions (see FIG. 3 a). 

1. An ORC apparatus for generation of energy by organic Rankine cycle, comprising: an organic working fluid; at least one heat exchanger (30) to exchange heat between a high temperature source and the working fluid, so as to heat and evaporate said working fluid; at least one turbine (40) fed with the vaporised working fluid coming out of the heat exchanger (30), to make a conversion of the thermal energy present in the working fluid into mechanical energy according to a Rankine cycle; at least one condenser (60) where the working fluid coming out of said at least one turbine (40) is condensed and sent to at least one pump; the working fluid being then fed to said at least one heat exchanger (30); characterised in that the heat exchanger (30) is of the hairpin type and comprises at least one inner tube (70) surrounded by an outer shell or jacket (80); wherein the inner tube (70) and outer shell (80) extend along at least two rectilinear stretches (70 b, 80 b) mutually connected by at least one curvilinear stretch (70 a, 80 a).
 2. An apparatus as claimed in claim 1, wherein the hairpin heat exchanger (30) is of countercurrent type, with or without buffers.
 3. An apparatus as claimed in claim 1, wherein the hairpin heat exchanger (30) comprises a single inner tube (70) surrounded by the outer shell (80).
 4. An apparatus as claimed in claim 1, wherein the hairpin heat exchanger (30) comprises a bundle of inner tubes surrounded by the outer shell (80).
 5. An apparatus as claimed in claim 1, further comprising at least one generator (50) operatively linked to said at least one turbine (40), wherein the mechanical energy produced by the turbine (40) is converted into electric energy.
 6. An ORC process for generation of energy by organic Rankine cycle, comprising: i) feeding an organic working fluid through at least one heat exchanger (30) to exchange heat between a high temperature source and said working fluid, so as to heat and evaporate said working fluid; ii) feeding the vaporised organic working fluid coming out of the heat exchanger (30) to at least one turbine (40) to make a conversion of the thermal energy present in the working fluid into mechanical energy according to a Rankine cycle; iii) feeding the organic working fluid coming out of said at least one turbine (40) to at least one condenser (60) where the working fluid is condensed; iv) sending the organic working fluid coming out of the condenser (60) to said at least one heat exchanger (30); characterised in that step i) comprises: making the organic working fluid flow through a heat exchanger (30) of the hairpin type comprising at least one inner tube (70) surrounded by an outer shell (80); wherein the inner tube (70) and outer shell (80) extend along at least two rectilinear stretches (70 b, 80 b) mutually connected by at least one curvilinear stretch (70 a, 80 a).
 7. A process as claimed in claim 6, wherein in step i) heating of the organic working fluid is of the super-critical type.
 8. A process as claimed in claim 6, wherein in step i) heating of the organic working fluid is of the sub-critical type.
 9. A process as claimed in claim 6, wherein the organic working fluid is selected from the group comprising: hydrocarbons, fluorocarbons and siloxanes.
 10. A process as claimed in claim 6, wherein the heat exchanger (30) of the hairpin type comes into operation in dry-running conditions. 